Question

In the figure, a smooth pulley of negligible weight is suspended by a spring balance. Weights of 1kg and 5kg are attached to the opposite ends of a string passing over the pulley and move with an acceleration because of gravity. During their motion, the spring balance reads a weight of:

• A. 6 kg
• B. less than 6 kg
• C. more than 6 kg
• D. may be more or less than 6kg

Answer 1

The correct option is B less than 6 kg

The Net weight of the system is 6kg.
If the system's centre of mass was stationary, The net upward force would have been 60N, and so the spring balance reading is 6kg.
But since the centre of mass of the system is accelerating downwards, there must be net downward force on the system, and so the upward force must be less than 60N.